1. Field of the Invention
The present invention relates to a tyre for vehicle wheels, as well as to a circumferentially substantially inextensible annular element—usually called “bead core”—incorporated in said tyre.
More particularly, the present invention relates to a tyre for vehicle, wheels comprising a bead core with improved characteristics as regards flexibility and ductility thereof.
2. Description of the Related Art
A tyre generally comprises: a carcass structure formed by at least one carcass ply, the ends of which are folded back or in any case fastened to substantially inextensible annular elements, i.e. the so-called “bead cores”; a tread band and a belt structure situated between the carcass and the tread band; and a pair of sidewalls applied to said carcass structure in axially opposite positions. The tyre portion which comprises the bead core is called “bead” and performs the function of fixing the tyre on the respective rim. According to a structure of the traditional type, in a position radically external to said bead core, the bead comprises a rubber strip, conventionally called “bead filling”, with a substantially triangular cross-section.
Generally, the rim on which the tyre is mounted comprises a central cylindrical well, from the axially opposite ends of which respective diverging surfaces extend axially outwards, said surfaces forming the “bead seats” of the tyre. Each of said surfaces terminates in a substantially vertical peripheral wall, so-called “flange” of the rim.
The function of the bead core is that of fixing the tyre on the rim and ensuring the transfer of forces and torques between tyre and rim.
The bead core, which is substantially inextensible in the circumferential direction, may be obtained from a single threadlike element, for example a steel wire, arranged in an annular configuration, or from a plurality of threadlike elements, for example several steel wires stranded together so as to define a cord or a plurality of cords, arranged in an annular configuration.
The internal diameter of the tyre bead (fitting diameter) coincides substantially with the diameter of the radially internal surface of the bead core, except for a difference between the two diameters due to the rubber coating of said threadlike element or plurality of threadlike elements forming the bead core itself, to the thickness of the folded-back portion of the carcass ply or plies, and to the presence of any elastomer reinforcing elements.
The diameter of the radially internal annular surface of the bead cores, and hence of the beads, of a tyre is smaller than the radially external diameter of the flange and is chosen so that, once the bead has been positioned in the respective bead seat of the rim, after passing over the flange, it is pushed by the pressure of the tyre inflating fluid along the diverging surface of the bead seat against the axially internal surface of the flange.
The fitting of a tyre on a respective rim is performed by using methods which are well known to the person skilled in the art.
In greater detail, said fitting operation starts with deformation (ovalisation) of a bead of the tyre so that, when the latter is positioned facing the rim, a portion of said bead is able to pass over the flange. Successively, the rest of said bead is also completely passed over the flange such that the bead is positioned in the closest bead seat. Then said bead is pushed axially towards the opposite bead seat so as to cause it to fall into the cylindrical central well of the rim. In this way, once said bead is located inside the abovementioned central well, the equatorial plane of the tyre may be inclined with respect to the equatorial plane of the rim so as to allow also the opposite bead to pass over the flange and be positioned in the corresponding bead seat, by means of ovalisation thereof (and hence of ovalisation of the respective bead core). Finally, the tyre is inflated so that both the beads come into abutment against the axially internal surfaces of the flange.
Owing to the rigidity of the bead cores, the fitting/removal operations of the tyre onto/from the rim require the use of levers with which it is possible to apply a force sufficient to deform the bead core, modifying the configuration from a substantially circular one to an oval one, so as to allow, as mentioned above, the bead to pass over the flange.
However, the use of levers acting on the threadlike elements forming the bead core may result in the elastic strain limits of said elements being exceeded locally. This fact is particularly undesirable since it may have a negative effect on the structural strength properties of the bead core during the travel of the tyre and, in some cases, may also result in breakage of one or more of said threadlike elements.
Different types of bead cores are known in the art.
For example, a typical bead core structure is the so-called “Alderfer” structure which has a configuration of the type “m×n”, where “m” indicates the number of axially adjacent threadlike elements or cords (obtained by stranding at least one pair of threadlike elements) and “n” indicates the number of radially superimposed layers of said threadlike elements (or said cords). This structure is obtained by using a rubberized strip comprising a predefined number of—textile or metallic—threadlike elements or cords and by, spirally winding (coiling) said rubberized strip onto itself so as to form a desired number of layers arranged radially superimposed one on top of the other. This constructional method allows the formation of cross-sectional contours of the bead core which are of a substantially quadrangular type. Examples of Alderfer structure are, in fact, 4×4, 5×5 or 4×5 structures.
A further conventional bead core structure consists in the so-called “single-thread bead core”. This is formed from a single rubberized threadlike element (or, single cord) which is wound spirally so as to form a first layer of axially adjacent turns; then, in a position radially external to said first layer, the same threadlike element (or the same cord) is further coiled so as to form a second layer in a position radially external to the first layer, and so on, so as to form several radially superimposed layers, each layer being able to have a number of turns different from that of the layers radially adjacent thereto. Therefore, by varying the number of turns in each layer, it is possible to obtain cross-sectional contours of the bead core with different geometrical forms, for example a hexagonal shaped cross-section. A regular hexagonal bead core may be formed, for example, by means of 19 windings arranged in the configuration: 3-4-5-4-3. This series of numbers indicates that the individual rubberized threadlike element (or single cord) is coiled so as to form firstly three turns axially adjacent to each other to form a first layer; then four turns axially adjacent to each other are provided in succession so as to form a second layer radially superimposed on the first layer, followed by five turns, axially adjacent to each other, so as to form a third layer radially superimposed on the second layer, then four turns axially adjacent to each other so as to form a fourth layer radially superimposed on the third layer and finally three turns axially adjacent to each other so as to form a fifth layer radially superimposed on the fourth layer.
A further conventional bead core structure is obtained by using a plurality of rubberized threadlike elements (or cords), each individual threadlike element (or cord) being radially coiled onto itself so as to form a column of radially superimposed wound turns. Several columns of turns, possibly with a different vertical extension (namely different number of wound turns radially superimposed on each other), axially adjacent to each other, thus form the abovementioned bead core. Preferably, the axially opposite end sections of one or more carcass plies of the tyre are arranged between one or more of said columns of wound turns and kept in the correct working position by said columns of wound turns. This type of bead core is described, for example, in the patent applications EP-943,421, EP-928,680 and WO 01/43957 in the name of the same Applicant.
So-called “twisted” bead cores are furthermore known in the art. This type of bead core has a central core, for example obtained from a single threadlike element which is welded end-to-end so as to form a circle, around which a threadlike element is spirally wound and finally joined to itself.
The number of turns performed by the threadlike element around the core, before joining, as well as the number of wires wound around the core and/or the number of crown configurations created around said core, determine the structural strength which is to be imparted to said bead core.
The twisted bead core is particularly advantageous with regard to the degree of flexibility which it is able to ensure, although manufacture thereof is particularly complex and therefore both time-consuming and costly.
As mentioned above, since the fitting/removal operations of the tyre onto/from the rim require deformation (ovalisation) of the bead core, in addition to the structural strength characteristics necessary for ensuring optimum anchoring of the tyre to the rim, the bead core must have an adequate flexibility so as to allow said operations to be performed as easily as possible and without causing permanent (plastic) deformation of the threadlike elements or some of them forming said bead core.
With reference to the types of bead core known in the art, as mentioned above, the flexibility characteristics may be varied by modifying the arrangement of the threadlike elements (or cords) in the various layers forming the bead core itself or, in the case of the twisted bead core, by modifying the relative arrangement of the core threadlike elements and the crown threadlike elements so that said threadlike elements contribute to the stress applied at varying degrees.
Moreover, the flexural rigidity of a bead core may be reduced by decreasing the flexural rigidity of the threadlike elements (or cords) which form it. In the case of a single threadlike element, since its flexural rigidity is proportional to the fourth power of its diameter, by reducing the diameter of the threadlike element it is possible to reduce the flexural rigidity. For example, the use of a single threadlike element with a diameter of 0.89 mm (using a high carbon content steel, with a minimum breaking load of 1350 N) instead of a single threadlike element with a diameter of 0.96 mm (using a standard carbon content steel, with a minimum breaking load of 1350 N) ensures the same structural strength of the bead core, but reduces the flexural rigidity thereof by 35%. At the same time, owing to the reduction in the diameter of the rubberized single threadlike element (and therefore of the total quantity of rubberizing compound used), it is possible to obtain a bead core structure which is lighter and more compact. The reduction in the diameter of the threadlike elements is frequently used in order to increase the flexibility of “Alderfer” and “single-thread” bead cores.
Among the bead cores mentioned above, the “twisted” bead core, because of its particular constructional characteristics, results in better fitting/removal operations of the tyre onto/from the rim since the central core ensures a final geometry of the bead core which is of an optimum nature, the overlying crowns ensure an excellent flexibility and, last but not least, the central-core/crown assembly guarantees the required structural strength.
Therefore, from that stated above it may be concluded that the characteristics required of the materials used in the production of a bead core, or rather of the threadlike elements forming the bead core, are numerous. In particular, during use, namely during the travelling of a vehicle, the main property required of a tyre bead core is the structural strength, to which the tensile strength (or resistance to tensile stress) of the threadlike elements forming said bead core contributes. Moreover, as mentioned above, during the conventional fitting/removal operations of a tyre onto/from a respective rim, the materials forming the bead core must have optimum flexibility and ductility characteristics.
However, such a combination of characteristics is not simple to be achieved since they are mutually exclusive in that the improvement of one of said characteristics results in a worsening of at least another one of said characteristics.
In the known art, in order to avoid permanent local deformations in the area of the bead (for example due to the use of levers during the fitting/removal operations of the tyre onto/from the rim), it has been proposed to subject the threadlike elements (or the cords) of the bead core to a heat treatment able to ensure an increase in the elongation at break of said threadlike elements (or cords).
However, said increase is obtained to the detriment of the tensile strength and the yield point which, due to said heat treatment, decrease significantly as shown by the load/deformation curves typical of a high carbon content steel traditionally used for the construction of bead cores. In fact, from said curves it can be determined that, with an elongation at break greater than, for example, 5%, the corresponding yield point is generally equal to about 0.20 (or less). The outcome of this, therefore, is that the material is subjected to a local plastic deformation which can no longer be recovered, this meaning that, once fitting of the tyre on the rim has been completed, the anchoring effect of the bead core on said rim will inevitably adversely affected.
An example of a bead core obtained from heat-treated cords is described in the document U.S. Pat. No. 5,702,546. In particular, said document describes a tyre, the carcass plies of which are not folded back around conventional bead cores, but are gripped and fastened between turns of cords arranged circumferentially so as to form several adjacent rows performing the function of bead cores. According to said document, the elastic and plastic elongation properties of said cords have been modified by means of heat treatment in order to obtain suitable tensile strength, flexibility and ductility values.
The Applicant has perceived that there is the need for a bead core for tyres to guarantee both a satisfactory structural strength such as to ensure an efficient anchoring of the tyre to the rim and an adequate flexibility such as to allow the bead core to be deformed elastically, for example during the fitting/removal operations of a tyre onto/from a respective rim.
In particular, the Applicant has perceived that a bead core for tyres must guarantee a satisfactory structural strength and a suitable flexibility in the case where the tyre is travelling in an at least partially deflated condition, for example due to a puncture.